1. Field of the Invention
The present invention relates to an image forming apparatus having an ACS (Auto Color Selection) mode for discriminating automatically whether a read document is in full color or monochromatic color and processing an image, and an image forming method.
2. Description of the Related Art
In a four-each tandem type image forming apparatus, image forming units composed of a photosensitive drum and a developing device are prepared for each of four colors such as yellow, magenta, cyan and black and are arranged along an intermediate transferring belt formed on an endless loop. Images in each color formed by each image forming unit are superimposed on the intermediate transferring belt, thus a full color image is formed. When performing monochrome print by the four-each tandem type image forming apparatus, either of the separation mode and contact mode is used.
In the separation mode, if only the black image forming unit is operated, the printing operation can be performed, so that the image forming units of the other three colors of yellow, magenta and cyan are stopped, and the three image forming units are separated from the intermediate transferring belt, thus the monochrome print is performed. In the contact mode, the image forming units of all the four colors including the image forming units of three colors which are not used actually for printing are kept in operation, are permitted to make contact with the intermediate transferring belt, thereby perform the monochrome print. Therefore, the image forming units of yellow, magenta and cyan perform the operation without performing an actual print. Further, at time of full color print, the image forming units of all the four colors are operated and similarly to the contact mode aforementioned, the image forming units of all the four colors make contact with the intermediate transferring belt, thereby performing the full color print.
At time of monochrome print, when printing in the separation mode, in a print job including both the full color print and monochrome print, the switching operation of the contact mode and separation mode is necessary. However, the switching operation cannot be performed instantaneously, and the switching requires a lot of time, so that continuous print cannot be performed, and the print performance is lowered. When printing in the contact mode at time of the monochrome print, since both the full color print and monochrome print are performed in the contact mode, no switching time is generated. However, in the contact mode, a problem arises that the photosensitive drums of yellow, magenta and cyan which do not print actually progress in wear or the developer progresses in deterioration due to charging.
To solve the two problems concerning the aforementioned print performance and apparatus life, an art of switching and using the contact mode and separation mode at time of the monochrome print is proposed. In the art described in Japanese Patent Application Publication No. 2003-237191, at time of the monochrome print, when the next image is monochromatic, it is printed in the separation mode. Further, the art generates a pattern with the information of the three pages of the preceding image, the image to be printed, and the next image and judges which is applicable, the separation mode or contact mode.
However, in the art described in Japanese Patent Application Publication No. 2003-237191, only images of the preceding and subsequent one page are taken into account at maximum and the performance required in the previous print is not taken into account. Therefore, as a whole, there are possibilities of a reduction in the performance.